Polymers that are used as binders in the preparation of coatings compositions usually require that a cross-linking reaction occurs after the application of the composition. This cross-linking reaction is necessary to obtain desired properties such as mechanical strength, resistance against chemical agents, exterior durability, etc. The cross-linking is often the result of the reaction between functional groups on the polymer and co-reactive functional groups on a cross-linker added to the composition. Examples are the reaction between the hydroxyl groups of a polymer and melamine-formaldehyde resins or between hydroxyl groups and polyisocyanate resins.
High performance, low temperature curing polyurethane or polyurea coatings include reactive polyisocyanates and active hydrogen-containing compounds such as hydroxyl-containing polymers or amine-containing polymers. Although these materials exhibit excellent performance and cure at low temperatures, the isocyanates may, under some conditions, be relatively hazardous to handle.
The coatings industry has been trying for years to develop non-isocyanate coating systems without sacrificing performance. Various non-isocyanate systems developed suffer from inferior performance in one or more areas such as durability, gloss retention, drying, hardness, solvent and humidity resistance and the like.
The market interest for non-isocyanate coatings is driven by environmental and safety concerns. Lower volatile organic compounds (VOC) and higher performance continue to be the driving forces for developing new and innovative coatings systems.
A non-isocyanate reactive coatings system, identified as the AA (All Acrylic) System, is available from Akzo Nobel Resins, a part of Akzo Nobel. This AA system is composed of two functional compounds: the first compound is a polymer containing epoxy and hydroxyl functionalities and the second compound is a polymer having tertiary amine and acid functionalities. The epoxy and tertiary-amine groups are the dominant functionalities in these polymers.
The first major cure reaction in AA is the reaction of t-amine with the epoxy group to form a quaternary ammonium ion with the aid of carboxylic acid:
Once the acid groups are consumed by the formation of quaternary ammonium ions, other reactions take place to consume the excess epoxy groups. For a description of the chemistry of “all acrylic” coatings see Leo G. J. Van de Ven, Rene T. M. Lejzer, Egbert Brinkman, and Paul Vandevoorde, Double Liasion, “Curing Mechanism of Waterborne Isocyanate Free All-Acrylic coatings”, No. 498–499, pp. 67–71 (1997); also see: Proceeding Eurocoat 97, September 23–25, pp. 549–560 (1997) and Farbe Lack, Vol. 105, No. 8, pp. 24–28 (1999),; E. Brinkman, and Paul Vandevoorde, “Waterborne two-pack isocynate-free systems for industrial coatings”, Progress in Organic Coatings 34, pp. 21–25 (1998); E. Manning and E. Brinkman, “All acrylic technology as an alternative to isocyanate-free polyurethane systems, Polymers Paint Colour, J. Vol. 190, No. 4426, pp. 21–23, (2000); Richard Hall and Maarten Weber, “Waterborne All Acrylic (WBAA) Coatings in Transport and its Structure”, Eurocoat, 2001, Lyon France.
The early hardness development (performance during the first few days), the ultimate flexibility, and solvent resistance of this system are unsatisfactory for applications in metal coatings, such as for small machinery, agricultural implements and construction equipment. Improved performance properties are desired. The present invention addresses these weaknesses.
Other non-isocyanate systems comprise various combinations of anhydride, hydroxyl, epoxy, and acid compounds. These formulations utilize tertiary-amine as a catalyst for the anhydride-hydroxyl reaction. The tertiary-amine catalyst does not contribute to the crosslinked structure of the film network. These systems have poor chemical resistance, hydrolytic instability and poor exterior durability.
The following patents are a few examples of the prior art where catalytic amounts of tertiary amines are used to catalyze the anhydride-hydroxyl reaction. U.S. Pat. No. 4,732,790 describes a process of coating where one of the compositions applied is a high solids coating based on hydroxy-functional epoxies and anhydride. U.S. Pat. No. 4,452,948 describes a two-pack coating system comprised of a hydroxyl component, an anhydride component and a catalyst. U.S. Pat. No. 4,871,806 refers to formulations of curable compositions comprising an acid-functional compound, an anhydride-functional compound, an epoxy-functional compound, and a hydroxy-functional compound. U.S. Pat. No. 5,227,243 refers to a substrate coated with a composition including a hydroxy-functional compound, an anhydride-functional compound and cycloaliphatic epoxy compound. U.S. Pat. Nos. 4,826,921, 4,946,744, and 4,798,745 refer to the formulation of compositions including anhydride and hydroxy-functional compounds. U.S. Pat. No. 5,602,274 refers to the use of non-cyclic anhydride with other coreactants such as, polyols, amines and epoxies. None of the prior art describe mixtures of compounds containing four (4) or five (5) functionalities in which a polymeric tertiary amine is used as co-reactant.